U.S. Pat. No. 4,798,048 granted to T. R. Clements on Jan. 17, 1989 entitled Augmentor Pilot and assigned to United Technologies Corporation, the assignee common to this patent application, discloses and claims a pilot embedded in the tail cone of the gas turbine engine. The pilot serves to create a recirculation zone for supporting combustion in the augmentor's burner section. Actuation of the augmentor initiates fuel flow from the spray bars which, in a well known manner, is ignited by an igniter or a flash of flame from the main engine burner. The tail cone, which is mounted on the rear end of the turbine exhaust case and extends into the augmentor is particularly and judiciously contoured to define a smooth aerodynamic diffuser and transition section between the main engine and augmentor to assure proper operation in the non-afterburning and afterburning operations. As disclosed in the aforementioned patent, the generally conically shaped tail cone fairing toward the apex extending axially to the aft end of the augmentor includes a step change at the base forming a shoulder which articulately defines the pilot section of the augmentor. Fuel from typical spray bars feed the pilot and the combination of the fuel and the recirculation of the engine's fluid working medium adjacent the pilot maintain a stable combustion process.
While the apparatus designed according to the teachings of the aforementioned patent is adequate for certain application it has for other applications proven to be inadequate. In particular, the recirculation pattern created by the step-change pilot introduces a condition of periodic violent pressure fluctuations generated by the unsteady release of energy during the afterburning combustion process occurring in the augmentor duct. This high frequency cyclic vibration, otherwise known as screech, can become so intense that it can reach destructive proportions. Additionally, screech under certain scenarios can cause rapid deterioration or failure of the flame holder. The typical method of preventing screech is by including a perforated liner internally mounted in the augmentor dust. The porosity i.e. the number of cooling feed holes in the linear are pre-ascertained to avoid the screech condition. Further, as is disclosed in U.S. Pat. No. 4,798,048 supra, includes means for separating the rear recirculation zone downstream of the pilot. As noted therein, this patent suggests the use of a jet of cooling air to be injected axially into the gas path for separating the recirculation zones. The cooling air forms a sheet of high velocity air for substantially preventing combustion in the downstream zone.
I have found that I can prevent screech from occurring in the pilot by including means other than a porous liner of the type discussed immediately above by the method of radially injecting a sheet of cooling air in the gas path. This invention contemplates introducing a second recirculation zone in juxtaposition to the heretofore known recirculation zone which is located downstream thereof as viewed relative to the direction of the engine's gas path by injecting an annular airstream of cooling air radially into the gas path. The tail cone is constructed with a base and conical portion where the two portions are joined by flanges arranged to provide means for radially injecting the circumferential air stream into the gas path.